1. Field of the Invention
The invention relates to a method of reducing the amount of hydrogen sulfide in an aqueous system, particularly in an aqueous system used in oil-field applications.
The invention further relates to a method of preventing sulfate-reducing bacteria from forming hydrogen sulfide in an aqueous system.
The invention further relates to a method of using an aqueous system which has a reduced amount of hydrogen sulfide and a reduced tendency to become contaminated with hydrogen sulfide in enhanced oil recovery.
2. Background of the Invention
Many oil reserves have turned sour due to the microbial production of hydrogen sulfide (H.sub.2 S). The hydrogen sulfide is produced by sulfate-reducing bacteria which convert sulfate in the system to sulfide. These bacteria arise during the drilling for oil and may also be indigenously present before the drilling. J. R. Postgate's book "The Sulphate-Reducing Bacteria", second edition, Cambridge University Press 1984, describes these bacteria and their affect on oil fields.
The hydrogen sulfide causes corrosion of the equipment used to recover the oil and can drastically damage the production capabilities of the oil field and also lowers the commercial value of the recovered crude oil. Accordingly, there has been intensive investigation directed at preventing the formation of hydrogen sulfide and/or removing the hydrogen sulfide once it is produced in oil fields.
For example, it is known that the addition of molybdates will inhibit and/or kill the sulfate-reducing bacteria (SRB) which are responsible for the production of hydrogen sulfide in natural environments, such as sediments. However, this method requires that vast amounts of molybdates, e.g., in excess of 3,000 ppm in the water to be treated, be used to effectively control the hydrogen sulfide production by SRB. The use of such large amount of molybdates has the associated disadvantages of high cost due to the limited availability of molybdates and lower efficiency in saline environments or in other brine environments such as connate waters.
Also it is known that the addition of nitrates to a system containing SRB will reduce the amount of SRB in the system and thus the amount of hydrogen sulfide formed by SRB. This method relies on strains of Thiobacillus denitrificans which are inhibited by organics which are present in oil field waters.
Accordingly, there is a need to provide an economical and effective means to remove hydrogen sulfide which is present in aqueous systems. There is further a need to prevent the formation of further hydrogen sulfide by sulfate-reducing bacteria which may already be present in the system and/or added later during the use of the system. There is further a need to provide an aqueous system which is useful in the recovery of oil which contains a reduced amount of hydrogen sulfide and furthermore will not be susceptible to forming hydrogen sulfide at a point later in the process, so that the system will not adversely affect the equipment used in the process. These and other needs have been solved by the present invention.